Such a pneumatic device is suited for the metered delivery of viscous, sticky or chemically aggressive liquids. Compared with a mechanical pump, it has the advantage that it is subjected to little wear and can be more easily cleaned. However, it has the disadvantage that the amount of liquid delivered is not determined volumetrically as is the case for example with the mechanical pump described in the European patent application EP 979028. The term “metered delivery” means that each time a predetermined amount or portion of liquid is delivered.
With the mounting of semiconductor chips, epoxy based adhesives are often used that contain flakes of silver. The adhesive is located in a syringe to which pressure pulses from the pneumatic device are applied in order to eject the adhesive in portions. Application of the adhesive is done either by means of a dispenser nozzle that has several openings through which the adhesive exudes and is deposited on the substrate, or by means of a writing nozzle that has one single opening through which the adhesive exudes. The writing nozzle is guided along a predetermined path by means of a drive system that can be moved in two horizontal directions so that the deposited adhesive forms a predetermined adhesive pattern on the substrate. A semiconductor mounting apparatus with a writing nozzle that can be moved in two horizontal directions is known for example from EP 1432013. Delivery of the adhesive is done in that the pneumatic device produces a pressure pulse that lasts just as long as the writing movement of the writing nozzle. Pneumatic devices are known from the patent specifications U.S. Pat. No. 5,199,607 and U.S. Pat. No. 5,277,333 that can be used for this application. These devices contain a pressure tank the pressure level of which is regulated by a pressure regulator. These devices have two significant disadvantages:                The amount of liquid delivered is dependent on the degree of emptying of the syringe. In order to keep the amount of delivered liquid constant, the length of the pressure pulse is varied. This hampers the co-ordination with the writing movement of the writing nozzle.        The pressure level of the pressure pulse changes during delivery of the liquid as the pressure regulator wants to compensate the pressure loss occurring in the pressure tank at the beginning of the pressure pulse.        